Conventionally, it is known that the air-fuel ratio feedback control is performed to correct the fuel injection amount so that the air-fuel ratio of an air-fuel mixture to be combusted in an internal combustion engine is equivalent to a target air-fuel ratio. In particular, when the air-fuel ratio feedback control is executed by means of a control system for an internal combustion engine which uses compressed natural gas (CNG), it is required that the internal combustion engine can be properly operated even when the property of CNG is changed. For this purpose, such a technique is known that a parameter, which relates to the combustion state of the air-fuel mixture, is corrected in accordance with the learning control on the basis of the magnitude of the correction value of the fuel injection amount in the air-fuel ratio feedback control after the charging is performed with fuel (see, for example, Patent Literature 1).
That is, in the case of the internal combustion engine which uses the fuel having a possibility that the property may change every time when the charging is performed, it has been feared that the engine stall or the misfire may arise if the same combustion condition is maintained, resulting from the fact that the stoichiometric air-fuel ratio is changed between the situations provided before and after the charging with the fuel. As a countermeasure thereagainst, the air-fuel ratio feedback control has been performed in some cases while corresponding to the difference in the fuel property by performing the learning correction control with a second correction value so that the magnitude of a first correction value is decreased if the first correction value, which is used in order to allow the air-fuel ratio of the mixture to approach the target air-fuel ratio in the air-fuel ratio feedback control, has the magnitude that becomes excessively large after the charging with the fuel.